JPS63223874A - Reproduction processing system for synthetic aperture radar picture - Google Patents

Abstract

PURPOSE:To estimate a frequency centering on Doppler by applying the function of a digital filter to complex number data obtained after multiplication of azimuth spot image patterns in an azimuth direction frequency area. CONSTITUTION:RAS reception data is compressed in the range direction by range compression processing 101. The initial value of a reproduction parameter is calculated by reproduction parameter calculation processing 102 and stored in a reproduction parameter file 103. Then the range compressed reception data is converted into a frequency range in the azimuth direction by azimuth direction FET processing 104 and then undergoes the correction of the range curvature distortions by range curvature correction processing 105. In azimuth spot image pattern production processing 106, a spot image pattern is produced with reference to the file 103 and converted into a frequency range via the process 107 to undergo complex multiplication 108 with the range curvature corrected data.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to synthetic aperture radar (Synthetic Aperture Radar) mounted on an artificial satellite or aircraft.
The present invention relates to a digital processing method for reproducing human-understandable images from imaging data obtained by Radar (hereinafter referred to as rSARJ), and particularly to a processing method suitable for reproducing high-quality images at high speed.

[Conventional technology]

In the field of moat sensing, using artificial satellites or aircraft, SAR is used as a sensor to image the ground surface, and can obtain high-resolution images in the microwave band that penetrates clouds.
is attracting attention.

Figure 4 shows the entire SAR system. Radar sensor 401. A SAR having an antenna 402 is mounted on an artificial satellite or the like and images the ground surface while moving in the direction of an arrow 404 on a flight path 403. Imaging data from the SAR is received by a ground station 405 and processed by a data processor 406 to create a video film 407, a magnetic tape 408 for data storage, and the like.

Note that 409 indicates a resolution cell, 410 indicates a range direction on the ground surface of data collected by SAR, 411 indicates an azimuth direction, 412 indicates an antenna beam, and 413 indicates a cutting width.

An overview of the processing of data collected by SAR will be described below. For more details, please refer to the Proceedings of the 13th International Symposium on Environmental Remote Sensing.

p, 337-360. April 1977 (Proceedings of 1 3 7h
Int, ernational Symposium
m on Remote Sensing of Env
ironmert, p, 337-360, April
il.

Benne and Cumming (1977)
“Digital Synthetic Frontage Radar Image Creation, Airborne and Satellite Results” (“Digital S A RIm
age Formation, Airborne
and 5atelliteResults”
).

In the received SAR image, one point on the original image is distributed with the spread of a point spread pattern h (t, r), and cannot be used as is. Here, r is the range direction, and t is the azimuth direction. It shows the direction. The information spread in the received image is first compressed in the range direction and then in the azimuth direction. Range compression processing is performed by correlation processing with point image pattern data for each line of image data. However, if the correlation process is executed as is, it will take a huge amount of processing time.
Fast Fourier transform (hereinafter referred to as rFFTJ), complex multiplication, fast inverse Fourier transform (hereinafter referred to as rIFFTJ),
) to speed up the process. To perform correlation processing using FFT, first, a point image pattern is generated by digital processing by a computer, and FFT of both the one point image pattern and one line of image data is calculated. Correlation of two data is a simple multiplication in the frequency domain after calculating FFT, so FFT of the above two data
By taking the product of the calculation results and applying IFFT to it,
A correlation result of 1542 minutes is obtained.

Similar to the above range compression, azimuth compression also uses FFT to obtain correlation results at high speed.

However, even if the speed is increased using FFT as described above, the amount of calculation required for SAR image reproduction processing is still enormous, and an ultra-high-speed calculation device is required to perform the processing within a practical time. It is known that

On the other hand, in order to obtain a high-quality SAR image, it is necessary to accurately estimate the point image pattern. In particular, the point image pattern in the azimuth direction depends on the moving speed of the SAR sensor and the distance from the ground surface, so it differs from scene to scene. It is possible to calculate the point image pattern in the azimuth direction from data on the position and velocity of the SAR sensor obtained by other means, but these data contain errors, and usually automatic estimation of the point image pattern from the imaging data itself is not possible. High-quality SAR images are often obtained using the focusing method.

The azimuth direction point image pattern h(t) is expressed as.

Here, t is the coordinate in the azimuth direction, and k.

β is a quantity called a reproduction parameter, k is a Doppler frequency change rate, and β is a Doppler center frequency.

A (t) is called an antenna pattern and is related to the intensity distribution in the azimuth direction of the microwaves emitted from the SAR sensor. In addition, β is estimated using completely different methods, but in particular, regarding the estimation of Doppler center frequency β, IE Transactions on Geoscience and Remote Sensing G.E. Number 1 (1985) pp. 47-56 (I E E E Transactions
on Geoscience andRensing
, GE-23, No. 1 (1985) Pp, 47-
56) “Doppler parameter estimation in satellite-borne synthetic frontage radar” by Li-(F, Li) et al.
D oppler P aramet, erEsti
mation for 5paceborne
Symthet, ic-A perture Rad
This method is described in a document titled "Ars").The method is to first reproduce an image using coarsely estimated reproduction parameters, and then Fourier transform the image data again in the azimuth direction to obtain a power spectrum. The Doppler center frequency is estimated from the shape.

According to this document, the center frequency can be estimated more accurately than the method of simply Fourier transforming the data after range compression in the azimuth direction and calculating the Doppler center frequency from the power spectrum.

[Problem that the invention seeks to solve]

However, according to the method in the above document, it is necessary to generate a point image pattern in the azimuth direction, perform FFT of the point spread pattern, multiply it with received data, and IFFT the result, and then perform FFT to obtain the power spectrum. However, there is a drawback that estimation of the Doppler center frequency requires a large amount of calculation and processing time, which accounts for about 10% or more of the entire SAR reproduction process.

An object of the present invention is to provide a SAR image reproduction processing method that improves the drawbacks of the above-mentioned conventional techniques, estimates Doppler center frequency accurately and at high speed, and enables reproduction of high-quality SAR images. .

[Means for solving problems]

The above purpose is 1. In normal SAR image reproduction processing, a digital filter is applied to complex number data after multiplication of an azimuth point image pattern in the azimuth direction frequency domain, and as a result, the power obtained as the square of the absolute value of the complex number data is This is achieved by estimating the Doppler center frequency using the spectrum.

[Effect]

Since filtering (convolution) in the frequency domain is equivalent to multiplication in the time domain, by selecting an appropriate filter, it is possible to achieve the same improvement in estimation accuracy as the removal of wraparound effects by the conventional method proposed by Lee et al. be able to. Furthermore, since the above effect is achieved by filtering in the frequency domain, it is possible to minimize the increase in the amount of computation required for estimating the center frequency and to perform estimation with high speed and accuracy.

〔Example〕

An embodiment of the present invention will be described below with reference to the flowchart of FIG.

In FIG. 1, SAR received data is first compressed in the range direction by range compression processing 101.

Next, the initial values of the playback parameters are calculated by the playback parameter calculation process 102, and the playback parameter file 10 is
The first-order range compressed reception data stored in 3 is converted into the frequency domain by FFT processing 104 in the azimuth direction, and then range curvature distortion is corrected by range curvature correction processing 105.

In the azimuth point image pattern generation process 106, a point image pattern is generated with reference to the reproduction parameter file 103,
After converting into the frequency domain in FFT processing 107, complex multiplication 108 with range curvature corrected data is performed. When control is first passed to the branch processing 113 for a certain scene, the process proceeds to the filtering processing 109 to remove wraparound effects, and then the contents of the reproduction parameter file 103 are updated based on the results of the Doppler center frequency estimation processing 110. do. Processes 106 to 108 are repeated using the contents of the updated playback parameter file, and when branching process 113 is reached for the second time, IFFT process 1 is executed.
11, the final SAR image is reproduced by multi-look addition/quantization processing 112.

The reproduced results are stored on magnetic tape or the like and distributed to users.

The filter used in the filtering process 109 is, for example, a three-point filter G + 1 = 0, 25-G o
=0.5. In the case of G1=0.25, the data after complex multiplication is Fk,=-Nd/2. ..., Nd/2-1 (Nd
: Number of processed data samples), then ~
1. Of course, this process can easily be implemented directly using software.

It can also be realized by dedicated hardware.

FIG. 2 shows how the wrap-around effect is removed in this case. It can be seen that the effect of wrap-around region 201 is controlled by filtering. Of course, increasing the length of the filter increases the amount of calculation, but it goes without saying that the wrap-around effect can be reduced more effectively.

FIG. 3 shows the principle of Doppler center frequency estimation processing 110. The horizontal axis 301 is the frequency in the azimuth direction, the vertical axis 302 is the power spectrum intensity, and the curve 303 is the power spectrum of the received data after filtering. Usually, the power spectra of multiple lines are averaged in order to reduce noise and improve estimation accuracy. Initial value of Doppler center frequency 3
04 is the value calculated in process 102 in FIG. 1. This value 304 is corrected to a value 305 that makes the left and right areas surrounded by the curve 303 and the horizontal axis 302 equal, and this is used as the estimated value of the Doppler center frequency. do. Here, we estimated the left and right areas to be equal, but of course the peak P of the curve 303
The effect of the present invention does not change even if the method of directly determining .

〔Effect of the invention〕

According to the present invention, since the wrap-around effect is removed by filtering in the azimuth direction frequency domain, the Doppler center frequency can be estimated at high speed and with high accuracy.
This has the effect that a high-quality SAR reproduced image can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a flowchart showing the processing procedure of an embodiment of the present invention, FIG. 2 is a diagram showing an example of removing the wrap-around effect in process 109 of FIG. 1, and FIG. 3 is a diagram showing the principle of process 110. , FIG. 4 is a block diagram showing the entire SAR system. 1st kindergarten f/ys ypr

Claims (1)

[Claims] In a synthetic aperture radar image reproduction processing method that estimates the Doppler center frequency from the azimuth direction power spectrum of received data, the power spectrum of the result of applying a digital filter to the data after multiplication of the azimuth point spread pattern in the frequency domain is obtained, and the A synthetic aperture radar image reproduction processing method characterized by estimating the Doppler center frequency from the shape of the power spectrum.